The invention relates to a method of linearizing variations in a transfer characteristic of a digital-to-analog (D/A) converter.
Commercial D/A converters exhibit differential variations in their transfer characteristics which result directly from the architecture of the converter. A D/A converter is disclosed by Frederick G. Weiss and Tyler G. Bowman in "A 14-Bit, 1 Gs/s DAC for Direct Digital Synthesis Applications," IEEE 13th Annual GaAsIC Symposium, Technical Digest 1991, Monterey, Calif., U.S.A., pp. 361-364, in which the four most significant bits (MSB) are encoded into a so called thermometer code. The least significant bits (LSB) are used in an uncoded state to quantize each segment or range by means of a R2R ladder network. A sum signal is simultaneously determined from weighted current sources and the segmented current sources (or encoded). However, differential variations in the transfer characteristic of the D/A converter occurring due to currents of different intensities in the segments of the thermometer code or the R2R network cannot be compensated in this way. Also, additional errors can result from delay times.
A D/A converter having high linearity is disclosed in WO 90/14717. In this case, errors arising during quantization range switchovers due to tolerances in components or processes are reduced by introducing fixed, predetermined switching thresholds. However, a prerequisite of this solution is at least two D/A converters of different resolution. Also, the converter output signal must undergo an evaluation, which can be skewed. As a result, this compensation scheme does not always function error-free.
A linearization method that likewise presupposes two D/A converters is disclosed in European Patent Application 430 449 A2 wherein a digital random number is summed with each sample of the digital signal and the sum is converted into analog form. The analog counterpart of the random number is subtracted from the analog sum to produce the analog counterpart of the digital signal.